Oxidation of ZrB2-TiC-Y2O3 at 1700°C

Abstract

The oxidation of plasma-sintered ZrB2-TiC-Y2O 3 samples in air and a CO/N2 atmosphere has been investigated at 1973 K. The TiC was added to develop a TiO2 similar to a SiO 2 layer, which has been an effective diffusional layer for the ZrB 2-SiO2 system. Yttria was added to determine whether sufficient stabilization of the tetragonal-ZrO2 (t-ZrO2) could occur to minimize the monoclinic-ZrO2 transformation, which causes susceptibility to thermal shock of any ZrO2 containing composite. ^ The oxidized samples developed an outer layer of TiO2-B 2O3 liquid acting as a sealant but with significant voidage as a result of B2O3 vaporization. Cracking initiated on the corners and then spread laterally for samples oxidized in air. For sampled oxidized in a CO/N2 atmosphere, the cracking was primarily found on the surface between the corners of the samples. The cracking may have resulted from cooling from 1973 K to room temperature as a result of the monoclinic transformation, but the TiO2 formation sufficiently slowed oxygen ingress to develop product layers. The product layers grew parabolically as an indication of a diffusional process resulting from oxygen ionic defects, which are readily known for the ZrO2-Y2O 3 system.^